Accurate assessments of O.sub.2 levels are essential for the effective control of fuel firing rate, ambient pressure, temperature and humidity, composition of fuel, stack draft and flow, and combustion air blower speed in combustion systems. With an O.sub.2 sensor as provided by this invention, increased efficiency and safety of the combustion system is significantly improved. In contrast to the present invention, conventional O.sub.2 sensors whose active sensor area is constructed from ZrO.sub.2 suffer from a number of drawbacks:
They utilize large electric heaters with power dissipation from 10 watts to 100 watts. This power is used to heat the sensor to an operating temperature of 800.degree. C.
Mechanical stresses induced by heating and by thermal expansion are large and lead to a short service life, and the voltage drop on the heater (if integral with the sensor) interferes with the measurement of the O.sub.2 signal.
The prior art employed many possible ways in which stabilized (cubic) zirconium oxide can serve as an oxygen sensor. The conventional types and electrode active types are well known in the art.
The pumping or cavity type and the two-or three-terminal type, with an active (redox) electrode of Pd, are able to measure oxygen without the need for a reference gas.
In another type, oxygen is pumped into and out of a small cavity (with or without a leak). This makes the fabrication of an absolute oxygen sensor possible without the need for additional metals like Pd.
Prior art solutions do not teach the combination of a bow-tie configuration in a zirconia based oxygen microsensor design wherein the zirconia film is supported by a Si.sub.3 N.sub.4 layer.
U.S. Pat. No. 4,587,105 to Bonne and Johnson teaches an oxygen sensor built on a silicon chip which has an SiO.sub.2 dielectric layer bridging over a depression in the surface of the chip. It does not teach the bow-tie configuration of the present invention.
U.S. Pat. No. 4,839,019 is directed to a limited current-type oxygen sensor comprising an oxygen ion conductive solid state electrolyte, a detection element including a positive electrode, and a negative electrode, and a heater element. The body is formed by a plurality of adjacent zirconium layers of approximately 100 micrometers in thickness.
U.S. Pat. No. 4,595,485 to Takahashi, et al. is directed to a limiting electric current type oxygen sensor comprising a first electrode of a gas-permeable film, a thin solid electrolyte film which is crystallized along one direction to decrease resistance and which has a thickness falling in the range between 0.1 micrometers and 30 micrometers, and a second electrode of gaspermeable film sequentially formed on an electrically insulating substrate. Takahashi, et al. teaches the use of a Pt-film as a heater in a zig-zag pattern. It does not teach the use of Si.sub.3 N.sub.4 for supporting the ZrO.sub.2 film, nor does it show a bow-tie configuration for the sensing device. Other differences are also evident from the figures.
U.S. Pat. No. 4,500,412 to Takahashi, et al. teaches an oxygen sensor with a heater. Takahashi, et al. uses a sensor which has an insulating substrate on which is formed a heater layer for heating the sensor on a part thereof and which is operated above a predetermined temperature. The heater layer is made of a material such as platinum, rhodium, palladium or a mixture thereof. The insulating substrate is made of aluminum, quartz, spinal, magnesia, zirconium or mixtures thereof.
U.S. Pat. No. 4,670,128 to Mase, et al. teaches a device including two electro-chemical cells, one serving as a sensing cell having first and second electrodes, and the other serving as a pumping cell having two electrodes, one of which is exposed to the measurement-gas space. Mase, et al. is directed to a laminar structure and does not use the etching approach of the instant invention.
U.S. Pat. No. 4,639,305 to Shibata, et al. is another example of an electro-chemical sensing element using a laminar structure.
U.S. Pat. No. 4,629,549 to Kojima, et al. teaches an oxygen sensor comprising a plurality of rectangular plates of different materials which uses a built-in oxygen sensor and is intended for use on exhaust pipes.
U.S. Pat. Nos. 4,559,126 and 4,880,519 are other examples of layered designs which have structures radially different from the instant invention.
U.S. Pat. No. 4,571,285 teaches an oxygen sensor for determining the partial pressure of oxygen in a monitor gas environment, including a diffusion housing of zirconium based material having a gas diffusion aperture, an oxygen ion conductive plate of zirconium based material, a pair of electrode layers mounted on opposite sides of the conductive plate, and a ceiling glass material bonding the conductive plate at one side to the housing to provide a diffusion chamber defined by the diffusion housing and the conductive plate.
U.S. Pat. No. 4,900,412 to Kerr, et al., teaches a solid electrolyte oxygen sensor using a heating sub-assembly. The device includes a substantially tubular solid electrolyte body having an elongated board centrally and axially located.
It is therefore the motive of the invention to provide a rugged O.sub.2 microsensor configured as a bow-tie and constructed from a suspended thin film of yttria-stabilized zirconia. The invention is useful for industrial and commercial combustion equipment. The invention can be used to sense low levels of excess air to help achieve maximum possible efficiency, safety or flue gas cleanliness.